1. Field of the Invention
In general, the invention relates to the monitoring of thermal neutron flux within a nuclear reactor, and in particular to an optical gamma thermometer for use in a monitoring string having a local power range monitor in which the measured temperature from the optical gamma thermometer, in conjunction with a steady-state heat balance, is used to calibrate the local power range monitor during its in-service lifetime.
2. Description of the Related Art
In the nuclear reaction interior of conventional boiling water reactors (BWR), it is possible to monitor the state of the reaction by either the measurement of thermal neutron flux, or alternatively gamma ray flux.
Thermal neutron flux is the preferred measurement. As it is directly proportional to power and provides for a prompt (instantaneous) signal from a fission chamber. The alternative measurement of gamma radiation does not have the required prompt response necessary for reactor safety requirements. Consequently, gamma radiation as measured by gamma thermometers is not used to measure and immediately control the state of a reaction in boiling water nuclear reactors.
Boiling water reactors have their thermal neutron flux monitored by local power range monitors, otherwise known as a local power range monitoring (LPRM) system. These local power range monitors include a cathode having fissionable material coated thereon. The fissionable material is usually a mixture of U235 and U234. The U235 is to provide a signal proportional to neutron flux and the U234 to lengthen the life of the detector. The thermal neutrons interact with the U235 and cause fission fragments to ionize an inert gas environment, typically argon, in the interior of the conventional local power range monitor. There results an electric charge flow between the anode and cathode with the resultant DC current. The amperage of the DC current indicates on a substantial real time basis the thermal neutron flux within the reactor core.
The boiling water reactor local power range monitors are inserted to the core of the reactor in strings. Each string extends vertically and typically has four spaced apart local power range monitors. Each detector is electrically connected for reading the thermal neutron flux in real time and for outputting the state of the reaction within the reactor. It is to be understood that a large reactor can have on the order of 30 to 70 such vertical strings with a total of about 120 to 280 local power range monitors. Such local power range monitors use finite amounts of U235 during their in-service life. Consequently, the sensitivity changes with exposure and they must be periodically calibrated.
Calibration is presently accomplished by using traversing in-core probes (TIPs). These traversing in-core probes are typically withdrawn from the reactor, as the traversing in-core probes are of the same basic construction as the local power range monitors and thus change their sensitivity with in-service life due to uranium 235 burnup.
In operation, the traversing in-core probes are typically calibrated. Such calibration includes inserting about five such probes separately to a common portion of a boiling water reactor. The boiling water reactor is operated at steady state and made the subject of an energy balance of a type well known in the art. The insertion of the traversing in-core probes occurs by placing the probes at an end of a semi-rigid cable and effecting the insertion within a tube system. Once a full core scan has occurred during steady state operation, a heat balance is utilized in combination with the readings of the traversing in-core probes to calibrate the local power range monitors.
In-core probes travel through the reactor in a specially designed tube system. This tube system constitutes through containment conduits into the interior of the reactor vessel. Into these conduits are placed semirigid cables which cables have the TIPs on the distal end thereof. The TIPs are driven into the drive tube system from large drive mechanisms and the entire system is controlled from an electronic drive control unit. The cables pass through so-called “shear valves” which valves can shear the cable and seal the conduit to prevent through the tube system leaks, which leaks may well be substantial before the cable and probes could be withdrawn. The cables further pass through stop valves admitting the traversing in-core probes to the interior of the vessel containment. Finally, the cables reach so-called indexers, and then to the interior of the reactor vessel. These indexers provide a mechanical system for routing each of the TIPs to pass adjacent the site of an assigned segment of the 170 some odd local power range monitors in a large boiling water nuclear reactor. It is normal for an indexer to include 10 alternative paths for a single traversing in-core probe to follow during a calibration procedure.
Needless to say, this system is elaborate and complex. Calibration of each local power range monitor is a function of the probe measurement of the local thermal neutron flux as well as a function of the position of the end of the inserting semi-rigid cable. Naturally, this position of the end of the semi-rigid cable has to be referenced to the proper alternative path for the necessary calibration to occur.
Further, the necessary tube system includes a matrix of tubes below the reactor vessel. Normally these tubes must be removed for required below vessel service and replaced thereafter.
Despite the presence of both stop valves and shear valves, the system remains as a possible escape route for water containing radioactive particles from the reactor. Further, the withdrawn cable can have mechanical complications as well as being radioactive.
For these reasons, it has recently been conceived to omit the use of the TIPs, and use, instead of the TIPs, another type of reactor power measurement apparatus in combination with the LPRM system. This type of apparatus, which is referred to as a gamma thermometer, comprises a system of sensors at a fixed position in the reactor that does not require a drive mechanism, nor does it involve substantial deterioration of sensitivity.
Gamma thermometers are known. In general, the gamma thermometer is a type of reactor power measurement apparatus, which detects the quantity of heat attributable to radiation, and in particular gamma rays. In contrast with a fission ionization chamber, the gamma thermometer does not, in principle, involve sensitivity deterioration.
Referring to FIG. 8, a typical gamma thermometer T is illustrated in a simplistic format. Typically, the gamma thermometer T includes a metal mass 74 suspended in a cantilevered fashion within an outer tube 76. The mass of metal 74 reaches a temperature, which is directly dependent on the gamma ray flux.
A reading thermocouple 78 and a reference thermocouple 80 are utilized in a series circuit. Specifically, the temperature differential between the reference thermocouple 80 (typically referenced to a temperature stable interior portion of the core) and the reading thermocouple 78 produce a voltage on paired lines 82, 84 which voltages indicate the gamma flux present which is proportional to reactor power.
Gamma thermometers are placed at fixed locations within the reactor. Each gamma thermometer requires an electrical connection and readout electronics. Unfortunately, gamma ray output as measured by gamma thermometers does not provide a prompt response to power transients as required for safe operation of the reactor. In addition, gamma thermometers are expensive and the probes and associated cabling occupy a significant amount of space within the reactor. Consequently, gamma thermometers are deployed in limited numbers and provide a coarser map of the reaction rate in the core than the TIP system. Therefore, it would be desirable to overcome the above-mentioned problems associated with gamma thermometers.